1. Field of the Invention
The present invention relates to an information recording medium utilizing a metallic recording layer. More particularly, the metallic recording layer comprises Bi and Se, with the recording material exhibiting excellent sensitivity, storage stability and archivability, as well as good signal-to-noise ratio.
2. Description of the Prior Art
Information storage systems in which information is stored by selective ablation, evaporation, removal or modification of a recording medium irradiated spotwise with a focused laser beam or the like having high density or strong energy are known as the so-called heat mode recording processes to those skilled in the art. The heat mode recording process is a dry process which does not need chemicals or treating solutions and in which a real-time recording can be made. According to this process, information can be quickly recorded in a high contrast form with a large capacity in respect to the amount of information recordable per unit area of recording medium, with the ability to add information later. Due to such advantages, the heat mode recording process is of wide application in a field in which a microimage recording medium, a computer output microfilm, a video disk, a computer signal memory medium or the like is involved.
The heat mode recording medium such as is used particularly as the video disk or the computer signal memory medium generally comprises an information recording layer formed on a suitable substrate such as a circular plate or disk of glass or a synthetic resin. When a pulse-modulated laser beam is focused on the recording layer rotated at a high speed, holes of about 0.6 to 1.5 microns in width are formed in the areas where the recording layer is irradiated with the laser beam, whereby information is recorded in the recording medium. The positions and sizes of the formed holes depend on the waveform of laser beam utilized, and correspond to the inputted information. The readout of the information stored in the recording medium is carried out by applying to the recording medium being rotated at a high speed a focused, weak laser beam whose intensity is lower than the threshhold value necessary to produce a hole in the recording layer, and hence detecting the variation of the reflected light.
The characteristics required of a heat mode recording medium are not only high sensitivity but also a high signal-to-noise ratio (hereinafter referred to as "S/N ratio"), storage stability, availability and the like. It is necessary for attaining a high S/N ratio in a video disk, a computer signal memory medium or the like, that the holes formed in the recording medium upon the selective ablation by a laser beam are substantially free of irregularities or disorders, and that the information readout light beam reflectivity or transmittance differences between the opening areas and the non-opening areas of the recording medium is as great as possible, which is equivalent to saying that the contrast is as high as possible. The terms "hole" and "opening" used herein are intended to mean ablated portions of the recording material in which information is stored.
Conventional recording media or materials used in heat mode recording or the like include those comprising a layer of an inorganic material, such as a metal or a metallic compound, and those comprising a layer of an organic material such as a dye, a pigment or a mixture thereof with a plastic. For recording information on such recording materials, the layer of the inorganic or organic material is irradiated with a laser beam to be evaporated or deformed in part to form a hole, or is caused to be subjected to a partial chemical change such as a change in degree of oxidation or in light absorptivity or reflectivity. In the case of a partial chemical change, the sensitivity of the recording material is generally so insufficient that the contrast between the changed areas and the unchanged areas is too low to obtain a high S/N ratio, and the archivability of the recorded material is too poor to be satisfactory in a practical sense. On the other hand, in the case of hole formation which is carried out by removing part of a recording layer provided on a substrate, in general, the recording material is relatively good but still unsatisfactory in a practical sense with regard to sensitivity and S/N ratio. The recording material, however, is defective in storage stability and archivability under a high humidity and in an oxidating atmosphere since its recording layer is exposed to the air for enabling the evaporation or removal of the recording layer. While the recording material used in a deformation-removal method is satisfactory in storage stability and archivability, it is unsatisfactory in sensitivity and S/N ratio.
Therefore, it is an object of the present invention to provide a heat mode recording material which not only has a high sensitivity but also is excellent in S/N ratio, storage stability and archivability.
It has been known to employ bismuth/selenium bi- or tri-layer systems as the recording layers in optical information media, i.e., media in which the information is recorded and/or read by optical means. The reason being that the formation of bismuth selenide from bismuth and selenium is a strong exothermic reaction. Thus, the efficiency of hole-machining by laser irradiation in thin bismuth films is increased by adding a selenium layer thereto due to the heat generated during the reaction of the bismuth with the selenide.
In Miller et al, "A Gallium-Arsenide Laser Facsimile Printer," Bell Systems Technology Journal, Vol. 58, p. 1909 (1979), there is described a two-layer film consisting of about 600 .ANG. of bismuth followed by approximately 650 .ANG. of selenium evaporated onto a low-surface-energy acrylic polymer substrate. This selenium thickness is anti-reflective at a thick cavity, GaAlAs, double-hetero junction laser wavelength (0.885 microns), permitting about 85% of the incident laser power to be absorbed in the bismuth layer, and the composition is nearly stoichiometric for Bi.sub.2 Se.sub.3. An exothermic reaction occurs when the energy delivered by the pulsed beam melts a disk of BiSe and the molten materials mix. The released energy, in combination with the increased absorption due to the selenium anti-reflective property, more than offsets the thermal mass added by the selenium. In fact, it is disclosed that a Bi/Se bilayer can achieve machining threshold at laser beam intensities four to six times lower than those required for bismuth alone.
Other disclosures of Bi/Se films useful as information recording layers include those in U.S. Pat. Nos. 4,000,492; 3,889,272; 3,911,444; 3,560,994; 4,290,075; 4,385,305 and 4,461,807.
Generally, however, the bilayer and trilayer bismuth/selenium films heretofore known suffer from a number of shortcomings when used in digital optical recording applications. They normally work in the transmission mode only. Excessive debris is formed around written holes, which results in a poor S/N ratio. For a given wavelength of a laser beam, the thickness is automatically fixed to satisfy an anti-reflective condition and the thickness ratio between Bi and Se is such as to satisfy the stoichiometry of Bi.sub.2 Se.sub.3. This does not allow one optimization of the thickness for the best hole-opening characteristics. As well, sensitivity can be improved.
Accordingly it is another object of the present invention to provide a Bi/Se recording layer for use in optical information recording applications which alleviates the aforementioned problems with conventional Bi/Se films.
Another object of the present invention is to provide a recording material comprising a Bi/Se recording layer which exhibits improved sensitivity as well as good S/N ratio, storage stability and archivability.
These and other objects, as well as the scope, nature and utilization of the invention, will be apparent to those skilled in the art from the following description and the appended claims.